1. Field of Technology
The present invention generally relates to optical systems. More specifically, the present invention relates to noise reduction in the transmission of optical signals.
2. Description of Background of Invention
Conventional data storage systems utilize billions of magnetically recorded imprints (bits) on a platter (media) surface to store oppositely polarized (e.g., positive or negative) data bits. These complimentary magnetic dipoles (which are parallel to the disk surface) represent a logic state of either a xe2x80x981xe2x80x99 or a xe2x80x980xe2x80x99. Based upon the industry""s current areal density (e.g., a few Gbits per in2) growth rate, such conventional disk drives are approaching areal densities as high as 20 Gbits/in2, which results in potential problems associated with a superparamagnetic limit. In particular, this physical limit causes oppositely-polarized domains that reside in very close proximity to one another to degrade, thereby causing data corruption problems.
To avoid this potential technological hurdle, an alternative storage technology utilizing a magneto-optical (MO) storage system is used. Such MO storage systems are, in principle, able to attain areal densities beyond approximately 40 Gbits/in2 without confronting the superparamagnetic limit. However, such an alternative technology results in the need to overcome new technological challenges, such as the effects of laser noise within the system. In particular, spectral polarization noise (SPN) comprising both mode partition noise (MPN) and laser phase noise must be minimized through careful optical system design.
For example, by propagating a multi-longitudinal mode laser light (e.g., from a Fabry-Perot diode laser) through a frequency selective polarization-maintaining (PM) fiber system, which contains slight unavoidable optical misalignment errors, SPN can develop, thereby increasing the likelihood of data corruption in a main light signal, which serves as the data conduit between an MO medium and a detection module. One partial solution for minimizing SPN is to utilize a single-mode (e.g., single-frequency) distributed feedback (DFB) laser, which does not generate these multiple modes within the system, thereby avoiding the effects of MPN. However, DFB lasers which operate in the red spectral range and at high power levels currently are not readily available on the commercial market. Although use of a DFB laser eliminates MPN, laser phase noise may still exit. In addition, since multimode laser diodes are considerably less expensive than DFB lasers, multimode lasers are the preferred type of laser source for MO storage systems.
What is needed is a system and method that utilizes a multimode diode laser and minimizes the effects of SPN within the MO storage system.
Accordingly, the present invention overcomes the deficiencies of the prior art by providing a system and method that minimizes the first-order spectral polarization noise (SPN) by time shifting polarization components of a parasitic light signal away from a main light signal. In particular, a preferred embodiment of the system includes a multimode laser, a leaky beam splitter (LBS), a first half wave plate (HWP1), a second half wave plate (HWP2), a polarimetric delay line (PDL), a polarization-maintaining (PM) fiber, a first quarter wave plate (QWP1), a second quarter wave plate (QWP2) and a differential detection module. A parasitic light signal is generated by non-ideal properties of the optical system.
The multimode laser generates the main light signal, which is used as a read signal for carrying the current logic state from a specific location on the MO medium to the differential detection module. The laser is modulated on and off at a radio frequency, the particular value of which is determined by the optical path lengths associated with the PDL and the PM fiber. The PDL and the PM fiber are part of a continuous birefringent optical conduit for the propagation of the main light signal to and from the MO medium.
The HWP1 and HWP2 in conjunction with the QWP1 alter the polarization of the main light signal to ensure that the first and second polarization components of the main light signal propagate along each delay path length of the PDL and each axis of the PM fiber. By propagating along one delay path length and axis on the forward path, and the opposing delay path length and axis on the return path from the MO medium, the two polarization components of the main light signal will have a net optical path difference of zero in the absence of an MO signal. In the presence of an MO signal, or magnetic Kerr effect, a small phase shift is introduced between the two polarization components of the main signal, making the net optical path difference slightly nonzero. To minimize SPN caused by retardation and/or orientation errors of QWP1, the PDL time shifts one half of the parasitic light signal ahead and the other half behind the main light signal so as to preclude coherent interaction between the parasitic and main optical pulse trains.
The LBS, which allows linearly polarized light to enter the PDL and the PM fiber on the forward path, reflects toward the differential detector on the return path, part of this polarized mode and most of the orthogonally polarized mode (generated by the magnetic Kerr effect) of the main signal. In addition, the LBS reflects a portion of the corresponding time-shifted parasitic signal toward the differential detection module. The QWP2 modifies the phase between the two polarization components of the reflected main light signal to ensure that the logic state of the data signal carried by the main light signal is properly detected by the differential detection module.